The present invention relates to an apparatus for and a method of checking the external appearance of a soldering state. More particularly, it relates to expedients for reliably and exactly measuring the shape of a solder fillet formed at the distal end of a lead, by the scanning of a laser beam.
Various expedients for checking the external appearance of a soldering state by the use of a laser beam have heretofore been proposed (refer to, for example, the official gazettes of Japanese Patent Applications Laid-open No. 177042/1988 and No. 177045/1988).
In any of the prior-art expedients, however, technical problems to be stated below, which are important for the check of the external appearance of a soldering state based on the projection of a laser beam remain unsolved.
Especially important in the soldering state is the shape of a solder fillet which is formed at the distal end of a lead. Whether the soldering state is accepted or rejected, is greatly affected by the quality of the shape of the solder fillet. In judging the acceptance or rejection of the soldering state, accordingly, naturally the laser beam must infallibly hit the solder fillet which is an object to be measured. In this regard, an electronic component placed on a circuit board involves a positional deviation, a dimensional error or the like relative to the designed position thereof. Therefore, even when the designed position is given by teaching beforehand so as to project the laser beam toward this designed position, the laser beam does not always hit the solder fillet. Particularly in recent years, the pitches of leads have a tendency to become very narrow in compliance with requirements for high densities of integration and packaging, and there are leads of small pitches less than 3 mm. When such a lead involves a slight positional deviation, even the projection of the laser beam toward the taught position results in irradiating a spot other than the solder fillet to-be-measured, and hence, an erroneous measured result is obtained.
Moreover, the solder fillet is made of a lustrous metal such as tin or lead and has a mirror-like surface, and it is substantially angled in its vertical section. Accordingly, unless a direction in which the laser beam is projected and a position at which light receiving means receives reflected light are contrived, the reflected light cannot be precisely received, so that a measured result becomes incorrect. It is the actual circumstances that expedients for reliably projecting the laser beam on the solder fillet and for reliably receiving the laser beam reflected by the solder fillet as described above have not been established yet.